An imaging diagnostic apparatus such as an X-ray computed tomography imaging apparatus scans a subject and generates medical image data. Contrast medium is often injected into the subject for image diagnosis of the subject's blood vessels. For example, an X-ray computed tomography imaging apparatus is used to describe blood vessels in CT images by scanning the subject in which contrast medium has been injected using X-rays.
In one application of the scan using a contrast medium and an X-ray computed tomography imaging apparatus, there is a developed function that monitors the degree of enhancement (e.g., the average CT value, etc.) in a region of interest (ROI) of a CT image. The function makes the apparatus execute a transition from a monitoring scan to a main scan according to the degree of enhancement. For example, the ROI is set in a region of a blood vessel on a CT image. The ROI is fixed on the CT image. Therefore, if the subject's body or organs move, the monitored region of the blood vessel might move to an area different from the ROI. In this case, the main scan might start at an incorrect time or might not start at all.